In addition, I have studied The LHC Inverse Problem and taken a few Adventures in Gravity. The articles below are listed in reverse chronological order by category, and those marked with a are recommended starting points to learn more about my work.
Jets are collimated sprays of particles arising from the fragmentation of short-distance quarks and gluons. In traditional collider studies, these jets are reconstructed using jet algorithms, which assign clusters of particles to jet four-vectors. I have shown that the substructure of jets can provide valuable information about the underlying short-distance physics. In extreme cases, physics that would otherwise be unobservable using traditional jet algorithms can be made prominent through jet substructure techniques.
Along with the rise of jet substructure tools, there has been an increased focus on using (non)perturbative QCD methods to predict the properties of jets. In the past, QCD calculations focused on “infrared and collinear safe” (IRC safe) observables which can be calculated order by order in perturbation theory. Now, the calculational toolbox has expanded to include IRC unsafe observables—such as track-based observables and ratio observables—by using new analytic techniques.
The boundary between jets and jet substructure has blurred over the years, and will continue to do so. The following techniques are primarily aimed applicable at identifying jets themselves, but take lessons learned from jet substructure studies to study the superstructure of events.
Dark matter is five times more abundant than ordinary (baryonic) matter, a fact that is firmly established through a variety of gravitational tests. A key question is whether dark matter might have other interactions with the standard model beyond gravity. One possibility is that dark matter experiences dark forces, which are only feebly felt by the standard model. A new generation of experiments is being pursued to find evidence for such dark forces, and I am involved in an MIT-led proposal called DarkLight to use the energy-recovery linac at Jefferson lab to search for a “dark photon”.
Dark forces are part of a large paradigm of dark portals connecting visible and hidden sectors of nature. I developed the idea of an “axion portal”, where dark matter and ordinary matter interact via a light pseudoscalar particle. While dark matter itself is quite difficult to probe in these scenarios, the axion leaves distinction signatures in collider experiments. Axion-like states and stable dark matter arise quite generically in supersymmetric hidden sectors, which can have an interesting effect on the measured cosmic ray spectrum.
One of key questions about dark matter is how it is produced in the early universe. In the standard weakly-interacting massive particle (WIMP) paradigm, dark matter is kept in thermal equilibrium when the universe is hot and dense, and once dark matter become sufficiently dilute from the expansion of the universe, it “freezes out” and the relic cold dark matter is what we observe today. However, there are many deviations from this standard paradigm, which lead to different predictions for the interactions of dark matter we can observe in direct and indirect detection experiments. Of particular interest is ultralight axion dark matter (not to be confused with the axion portal above), which requires very different detection techniques than WIMPs.
Supersymmetry (SUSY) is well-motivation extension of space-time symmetry, with unique predictions for collider experiments like the LHC. SUSY is particularly interest in the context of cosmology, since the geometry of the universe is de Sitter (dS), but supergravity (SUGRA) retains a memory of an underlying anti-de Sitter (AdS) algebra. I have studied implications this AdS structure, in particular showing that the phenomenon known as “anomaly-mediatied SUSY breaking” does not actually break SUSY, but is rather a SUSY-preserving effect in anti-de Sitter space.
If SUSY is symmetry of nature, then it must be spontaneously broken. However, very little is known about the dynamics of SUSY breaking, and it is possible (even likely?) that SUSY is broken independently by multiple sectors. In the familiar case where SUSY is broken by a single sector, this gives rise to a single goldstino (a Nambu-Goldstone fermion from SUSY breaking) which is eaten to form the longitudinal components of the gravitino. But if there are multiple sectors that break SUSY, then there is a corresponding multiplicity of “goldstini”, which can affect collider physics and cosmology.
In particle physics, the principle of naturalness states that measured parameters should be closely related to fundamental parameters. Unnatural theories exhibit delicate cancelations between fundamental parameters to yield anomalously small measured parameters. While naturalness is not a sacred principle in particle physics, it is the basis for much theoretical speculation. Naturalness features prominently in supersymmetric scenarios, but interesting theoretical ideas can arise if naturalness is abandoned completely or taken to logical extremes.
The origin of electroweak symmetry breaking is a key question in and beyond the standard model. In the standard model, the Higgs boson plays a key role in breaking electroweak symmetry, but this is not the only option. Models like technicolor invoke strong dynamics to break electroweak symmetry, and there is an intermediate possibility that strong dynamics yields a composite Higgs boson which subsequently breaks electroweak symmetry at a lower energy. One of the most interesting kinds of composite Higgs theories are “little Higgs” models, which invoke a special pattern of overlapping symmetries.
In addition to their phenomenological relevance, little Higgs theories offer an interesting testing ground to study spontaneous symmetry breaking itself. The following theoretical studies are aimed at gaining a better of understanding of the dynamics of composite theories, and the plausibility of various symmetry breaking patterns.
Motivated in part by compositeness models, I have studied a variety of non-supersymmetric theories that stretch the notions of naturalness. These models have revealed new ways to think about flavor physics, Higgs physics, and dark matter, as well as suggested new ways to search for top partners at the LHC.
With all eyes on the Large Hadron Collider (LHC), particle physics is posed to learn what physics lies beyond the standard model. After a discovery of new particles or new phenomena, the next task is to figure out how that discovery fits into a theoretical framework. Over the years it has become clear that very different theoretical models can give rise to very similar LHC signatures, a challenge dubbed the “LHC Inverse Problem”.
The excitement surrounding the Large Hadron Collider is focused on the possibility of discovering new physics beyond the standard model. However, in order to discover new physics signals, one must have a thorough understanding of standard model background. The workhorse tools for understanding background processes are parton shower generators, which simulate collider events through a variety of controlled and uncontrolled approximations. The goal of the GenEvA project (GENerate EVents Analytically) is to improve the approximations used in Monte Carlo programs as well as smoothly interpolate between different approximation method to construct as complete a picture of the standard model as possible.
Lorentz symmetry is the symmetry between space and time, and it is the basis for all fundamental physics. In particular, the absence of any “luminiferous ether” is well-established, and we know that all particles travel according to relativistic dynamics to an excellent approximation. However, there is still the possibility that Lorentz symmetry is spontaneously broken at low energies, and there are a variety of ongoing searches for this “new ether”, which has subtle effects on standard model properties.
One of the most exciting possibilities for the Large Hadron Collider (LHC) is the production of microscopic black holes. This is possible in models of extra dimensions, where the fundamental gravity scale is near the LHC center-of-mass energy. A key question is whether the semi-classical intuition for the black hole production rate is correct in the full quantum picture, and we were able to answer this affirmatively in a simple toy theory.